Miniature, high efficiency dual frequency ultrasonic transducer with selectable beamwidth

ABSTRACT

Ultrasonic transducer with a selectable beamwidth comprising a body of piezoelectric material in the form of an annulus having an outer diameter D and a thickness T. The body has proximal and distal generally planar parallel surfaces and a centrally disposed hole extending therethrough. The body has a cylindrical wall with a width W extending from the hole to the outer diameter. The transducer is capable of operating at low and high resonance frequencies with the low frequency resonance being determined by the diameter D and an aspect ratio of D/T and the high frequency resonance being determined by the thickness T and an aspect ratio of W/T.

This is a division of Ser. No. 08/759,054, filed Dec. 2, 1996, now U.S.Pat. No. 5,740,596 which was a continuation of Ser. No. 08/447,993,filed May 23, 1995, U.S. Pat. No. 5,581,144, which was a continuation ofSer. No. 08/178,081, filed Jan. 6, 1994, abandoned.

This invention relates to a miniature, high efficiency dual frequencyultrasonic transducer with selectable beamwidth, an assembly thereof andmethod.

In U.S. Pat. No. 5,059,851, there is disclosed a miniature ultrasoundtransducer assembly which utilizes a transducer in the form of a diskwithout a hole. With such a disk, it is known that there are two strongresonances, one related to the lateral (diameter) resonance and onerelated to the thickness resonance. In such transducers, however, sincethe thickness is close to the diameter (typically, the diameter wasapproximately twice the thickness), there was a strong interactionbetween the diameter and thickness resonances. Attempts to optimizeefficiency between the two resonances were found to be difficult toachieve because of the undesired coupling between the two resonancemodes which caused inefficiencies. FIGS. 3 and 4 of U.S. Pat. No.5,059,851 show a doughnut-shaped transducer. The specification suggestsan appropriate aspect ratio of 0.5-to-1. The distance from the outercircumference to the outer margin of the hole would be approximatelyone-fourth to one-third of the width extending across the entiredoughnut-shaped transducer. At the time of the disclosure in FIGS. 3 and4, certain advantageous features of such a construction were notappreciated. There is therefore a need for a new and improved ultrasonictransducer, assembly thereof and method which takes full advantage ofthe characteristics of such a transducer.

In general, it is an object of the present invention to provide anultrasonic transducer, assembly thereof and method which is capable ofdual frequency operation providing selectable beamwidth.

Another object of the invention is to provide a transducer of the abovecharacter which although of miniature size is of high efficiency.

Another object of the invention is to provide a transducer and method ofthe above character in which efficient broadband operation can beachieved.

Another object of the invention is to provide a transducer assembly andmethod of the above character which is particularly applicable tointravascular Doppler transducers for coronary or cerebral applications.

Another object of the invention is to provide a transducer assembly andmethod of the above character which is capable of operating in the 5-20MHz range.

Additional objects and features of the invention will appear from thefollowing description in which the preferred embodiment is set forth indetail in conjunction with the accompanying drawings.

FIG. 1 is an enlarged cross-sectional view of a transducer and anassembly thereof incorporating the present invention.

FIG. 2 is an end elevational view looking along the line 2--2 of FIG. 1.

FIG. 3 is an enlarged cross-sectional view of the transducer shown inFIG. 1.

FIG. 4 is an enlarged isometric view of the transducer shown in FIG. 3.

FIG. 5 is a block diagram and a schematic illustration showing themanner in which selectable beam width can be achieved utilizing thetransducer shown in FIGS. 1 through 4.

The ultrasonic transducer of the present invention is comprised of abody of piezoelectric material in the form of an annulus having acentrally disposed hole extending therethrough. It has an outer diameterD and proximal and distal generally planar surfaces. An electrode coverseach of the proximal and distal planar surfaces. The piezoelectricmaterial also has a thickness T extending from the proximal to thedistal surfaces and a wall width W extending from the outer margin ofthe hole to the outer diameter. The body is capable of operating at alow resonance frequency and a high resonance frequency. The lowfrequency resonance is principally determined by the diameter D and theaspect ratio of D/T. The high frequency resonance is principallydetermined by the thickness T and the aspect ratio of W/T.

More in particular, the transducer assembly 11 incorporating the presentinvention forms a part of a guide wire 12 which is of the type describedin U.S. Pat. No. 5,059,851. This guide wire 12 as described therein istypically comprised of a flexible elongate member in the form of astainless steel tube having a suitable length as for example 150 cm.This flexible elongate member can have a suitable diameter ranging from0.018" to 0.010". The flexible elongate member is provided with apassageway extending the length thereof. The distal extremity of theflexible elongate member may typically be secured to a coil spring andthe coil spring is secured to a cylindrical tip 16 which as describedtherein may be secured to the distal extremity of the spring by havingthe coil spring threaded onto the tip 16. The tip 16 is provided with acup-shaped cylindrical recess 17 which receives the transducer 21 of thepresent invention.

The transducer 21 is formed of a body 22 of a material of the typedescribed in U.S. Pat. No. 5,059,851 as for example a piezoelectricmaterial suitable for use as an ultrasonic as for example apiezoelectric ceramic. One piezoelectric ceramic found to beparticularly satisfactory is EC-98 lead magnesium niobate available fromEDO Corporation, Wester Division, Ceramics Division, 2645 South 300West, Salt Lake City, Utah 84115. The EC-98 composition was selectedbecause it provides a higher dielectric constant, low agingcharacteristics, excellent coupling characteristics and a high strainconstant which makes it particularly suitable for miniature transducersof the present invention.

The body 24 which has been formed as hereinafter described has the formof an annulus or ring and can be characterized as being doughnut shaped.The body 24 has substantially planar proximal and distal parallelsurfaces 26 and 27 with the distal surface 27 facing distally oroutwardly from the distal extremity of the guide wire 12 as shown inFIG. 1. The body 24 is provided with an outer cylindrical surface 28which extends in a direction generally perpendicular to the planarsurfaces 26 and 27. The body 24 is also provided with a centrallydisposed hole 29 which extends through the surfaces 26 and 27 atsubstantially right angles thereto.

The transducer assembly 11 is particularly constructed to be used insmall-diameter guide wires as for example those ranging below 0.018" andbelow. The transducer 21 therefore must have correspondingly smalldimensions so they can be utilized in the distal extremities of suchguide wires.

In order to achieve reliable manufacture of such transducers, it hasbeen found desirable to utilize laser machining. Such laser machiningforms cuts and holes with slight tapers, as for example 5° or less whichare shown in exaggerated form in FIG. 3. In connection with the presentinvention to provide such transducers 21, a sheet (not shown) of thepiezoelectric material having the desired thickness as for example 3.8mils is used. The transducers 21 are formed by a step-and-repeat processfrom the sheet of material. The amount of taper is determined by thetype of laser being utilized and the focal length of the objective lensfor the laser beam. By providing a lens having an increased focal lengthit is possible to reduce the taper. By utilizing an X-Y motion table inconjunction with a YAG laser, it has been found that it is possible toprovide a laser having a spot size ranging from 0.8 to 1.0 mil andhaving a pulse repetition rate ranging from 50 to 150 Hz to cut both theoutside diameter and the inside diameter for the transducer to providethe cylindrical body 22 with the hole 29 extending centrallytherethrough. Good results were achieved utilizing a YAG laser having anoperating frequency of 1064 nanometers.

It should be appreciated that other types of lasers can be used. Forexample, an excimer laser can be utilized and may be desirable becauseit has a low thermal distortion but the cut rates are less than thatwhich can be accomplished with a YAG laser. The excimer laser typicallyoperates at 308 nm.

With the use of the YAG laser, it is possible to produce the transducers21 at a relatively rapid production rate with minimal damage to thecrystalline structure. It has been found that any material evaporatedonto the surfaces 26 and 27 of the body 22 during the laser machiningoperation can be readily removed with a suitable solvent such as acetoneor alcohol utilizing a Q-tip. Thus there remains a very small heataffected zone near the outer perimeter of the hole 29 adjacent thecylindrical surface 28 and the inner perimeter adjacent the hole 29.

In order for the transducer 21 to fit within the cylindrical recess 17of the tip 16, the transducer 21 should have an appropriate diameter.Thus, by way of example, for a guide wire having an outside diameter of0.014", the transducer 21 should have an outside diameter ofapproximately 0.010". Using laser machining for such a size transducerit has been possible to achieve the cylindrical surfaces 28 and the hole29 in the surfaces 26 and 27 with a taper of approximately 5°. Thistaper can be improved to approximately 2-3° with an objective lenshaving a longer focal length.

In connection with the present invention of a guide wire 12 of the typeshown in FIG. 1, it is desirable that the transducer 21 produce a broadbeam to make it possible to cover as wide a cross-section as possible ofthe vessel in which it is disposed, and preferably to extend to the sidewalls of the vessel so that it is ensured that the velocity of theliquid, as for example blood, flowing through center of the lumen isaccurately measured by ensuring that the beam covers at least the centerof the vessel lumen.

As mentioned previously, the preferred sizes for intravascularguidewires are in the range of 0.010" to 0.018" diameter, with some aslarge as 0.030" or greater. The optimum Doppler frequencies forintravascular operations (considering attenuation, backscatterefficiency, scanning distance, etc.) are typically in the range of 5-20MHz (corresponding to acoustic wavelengths in blood of 0.003 inch to0.012 inch). These ranges for transducer size and ultrasound wavelengthcorrespond to acoustic transducers having diameters ranging fromapproximately 1 to 10 wavelengths, with the most desirable combinationsbeing transducers having diameters in the range of 1 to 5 wavelengths.

In connection with the present invention, it has been found to bedesirable to utilize the doughnut-shaped or ring-type transducer 21which has the capability of providing two strong resonant frequencies,one of which can be characterized as high-resonant frequency and theother of which can be characterized as a low-resonant frequency.

The low resonant frequency is strongly related to the largest dimensionof the transducer 21 which is the outside diameter D. The resonance canbe characterized by its frequency and efficiency. These parameters canbe predicted using a standard model for disk-shaped transducers. Forexample, Kunkel et al. in Finite-Element Analysis of Vibrational Modesin Piezoelectric Ceramic Disks, IEEE, Vol. 37, No. 4, July 1990 usefinite element analysis to calculate the normalized resonance frequencyand the electromechanical coupling coefficient (which is an importantelement of transducer efficiency) as a function of the diameter tothickness aspect ratio DIT of a ceramic disk transducer. The diameter Dcan then be used to transform the normalized resonance frequency to anactual resonance frequency. The centrally disposed hole 29 has only asmall effect on the frequency and electromechanical coupling coefficientof the fundamental low frequency mode, but it does act to suppress manyof the undesirable disk modes which would otherwise waste energy andthereby reduce the efficiency of the transducer. Thus the ring-typetransducer offers the possibility of an efficient low frequencytransducer, with a very pure resonant mode and weak harmonics.

The second or higher frequency resonant mode is strongly related to thecross section of the doughnut-shaped or ring-type transducer. Theresonance can be characterized by its frequency and efficiency. Theseparameters can be predicted using a standard model for lineartransducers of rectangular cross section, since the ring-type transducercan be envisioned as a linear element bent into a ring shape. Mason inPhysical Acoustics Principles and Methods, Academic Press, Vol. I, PartA, 1964 used analytical techniques to calculate the normalized resonancefrequency and electromechanical coupling coefficient (which is animportant element of transducer efficiency), as a function of the widthto thickness aspect ratio W/T of the ring cross section. The thickness Tcan then be used to transform the normalized resonance frequency into anactual resonance frequency. For width to thickness aspect ratios lessthan 1, the fundamental resonance mode for a rectangular cross sectionis often referred to as the length extensional mode. Theory predicts,and experiments confirm that the electromechanical coupling coefficientis maximized for a width to thickness aspect ratio of approximately 0.6,and this coupling coefficient is significantly higher than that found ina conventional thickness mode resonance transducer.

Proper choice of the three key transducer dimensions, diameter D,thickness T, and annular width W permits the efficient operation of aminiature transducer at two distinct frequencies. The centrally disposedhole suppresses the unwanted disk resonance modes, and permits efficientoperation using a low frequency lateral resonant mode. At the same timethe centrally disposed hole permits the transducer to take advantage ofthe efficient length expander mode of vibration in the thicknessresonant mode, with the aspect ratio W/T close to the theoreticaloptimum of approximately 0.6.

In the past in connection with transducers it was typically desired toprovide a transducer which has a single mode of operation with the otherfrequency modes being unwanted. In the present application, the presenttransducer takes advantage of an unwanted mode by creating alow-frequency lateral mode to provide a transducer which has two strongmodes and wherein other spurious modes have been limited to provide adual-frequency transducer to optimize the operation of the guide wire 12as hereinafter described. It should be appreciated that either one ofthe two resonant frequencies may be chosen to be optimized to providethe best performance for a specific application at the expense offoregoing the flexibility of dual frequency operation. It can be furtherappreciated that if the high and low resonant frequencies are designedto be close to one another, the two resonances will blend together toprovide a single broad resonance, thereby providing an efficientbroadband transducer design.

By way of example, a transducer 21 incorporating the present inventionhad a diameter D extending across the distal surface 27 of 10.2 mils anda diameter D extending along the proximal surface 26 of 9.4 mils. It hada thickness T of 3.8 mils. The hole 29 had a diameter at the proximalsurface 26 of 3.3 mils and a diameter at the distal surface 27 of 2.5mils. With such dimensions, the distance W from the outer margin of thehole 29 to the outer surface 28 was 3.85 mils at the distal surface 27and 3.0 mils at the proximal surface 26. The transducer 21 was mountedin the cup-shaped cylindrical recess 17 by suitable means such as amedical-grade adhesive of the type disclosed in U.S. Pat. No. 5,059,851.Also a matching layer 32 was provided at the distal surface 27 of asuitable material such as described in U.S. Pat. No. 5,059,851 toprovide a flush surface and to fill the recess 17 as shown particularlyin FIG. 1. Such a transducer 21 was found to have a low-frequency modeof operation of approximately 6.5 MHz and a high-frequency resonance of15.0 MHz. For other dual frequencies of 6.0 MHz and 10.0 MHz thetransducer 21 would have a diameter D extending across the distalsurface, would be approximately 10.8 mils and the thickness T would beapproximately 6.7 mils. For dual frequencies of 6.0 MHz and 12.0 MHz,the transducer 21 would have a diameter D extending across the distalsurface, would be approximately 10.5 mils and the thickness T would beapproximately 5.2 mils.

The transducer 21 was activated at these frequencies by a conventionalpower supply, transmitter and receiver represented by the block 36 inFIG. 5. The block 36 was provided with a frequency select switch 37which has the capability of selecting various frequencies as for examplea specific low frequency or a specific high frequency from a range offrequencies as for example 5 Mhz, 6 MHz, 7.5 MHz, 10 MHz, 12 MHz, 15 MHzand 20 MHz. These selected frequencies were applied to a pair of lines38 and 44 connected to a pair of insulated conductors 39 and 42. Theconductor 39 extends through the hole 29 and was bent over and solderedto the distal surface 27 of the transducer 21 as shown in FIG. 3. Theconductor 39 was provided with an insulating covering 41. The conductor42 was provided and had its distal extremity bent and soldered to theproximal surface 26 of the transducer 21. It was also provided with aninsulating covering 43. The Doppler transmitter from the block 36supplied pulses of electrical energy to the ultrasonic transducer 21which produced ultrasonic pulses that were propagated outwardly from thedistal surface 27 in a forwardly extending conically-shaped beam 46 inwhich the conical beam subtended an angle or had a beam width ofapproximately 30° to analyze a sample volume 47 at a suitable distanceas for example approximately 5 mm from the distal surface 27. When thetransducer 21 was excited at the low frequency, a conical-shaped beam 48was propagated forwardly and subtended an angle or had a beam width ofapproximately 60° to analyze a sample volume 49 as shown in FIG. 5 at adistance of approximately 10 mm from the distal surface 27. In general,the width of the beam from a small ultrasonic transducer (expressed inradians is λ/D, where λ is the wavelength of the ultrasound and D is thetransducer diameter, a lower frequency (with its corresponding longerwavelength) will provide a proportionally broader beam compared tohigher frequency operation.

In making Doppler blood flow measurements with the guide wire 12 havinga transducer assembly 11 of the present invention mounted thereon, it isimportant that the beam propagated by the transducer covers at least thecentral region of the vessel to properly ascertain spatial peak bloodflow, because it is in this region that the blood is flowing at thefastest rate. Thus in order to be sure that the proper blood flow isbeing measured, it may be desirable to utilize the lower operatingfrequency with its corresponding wider beam to ensure that substantiallyall of the cross-sectional area of a measurement location of a vessel,or at least a substantial portion of the same which includes the centralregion of the vessel is covered by the ultrasound beam. However, if theultrasound beam is much larger than the vessel, a great deal of acousticenergy will be lost as the beam spreads beyond the vessel walls. Thus inorder to ensure that the Doppler signal is as strong as possible, it maybe desirable to utilize the higher operating frequency with itscorresponding narrower beam as long as the beam is large enough to covera substantial portion of the vessel. For example, with a large vessel asfor example a femoral artery having a 10 mm diameter, there is anexcellent opportunity to pick up the central flow in the artery with thewider beam 48, whereas this might be difficult to do with the smallerbeam 46. Conversely, in the case of a small vessel as for example acoronary artery having a diameter of 3 mm, the narrower beam 46 will beadequate to cover a substantial portion of the vessel, whereas the widerbeam 48 might provide a weak Doppler signal. Thus, with the presentinvention, it is possible to make accurate measurements in a wide rangeof vessel sizes even though the distal extremity of the guide wire maybe bent to accommodate passing into side branches and tortuous vesselsand thus may not be centered in the vessel.

From the foregoing it can be seen that there has been provided atransducer assembly for use on a guide wire which is particularlyadapted to making flow measurements in a vessel in a body as for examplethe human body. The transducer is one which has dual-frequency operationwith high and low resonance frequencies which are substantially equallyefficient. By utilizing such a transducer, a method can be utilized toprovide narrow and wide beams to ensure that at least the center of thevessel where maximum flow occurs will be in the sample volume beingmeasured.

What is claimed is:
 1. An ultrasonic transducer capable of operation ata low frequency resonance and at a high frequency resonance, comprisingan annular body of piezoelectric material with an axially extendinghole, an outer diameter D, a thickness T, and a width W extending fromthe hole to the outer diameter, the low frequency resonance beingdetermined by the diameter D and an aspect ratio D/T, and the highfrequency resonance being determined by the thickness T and an aspectratio W/T.
 2. The transducer of claim 1 wherein the body has a generallycylindrical outer surface and generally planar proximal and distal endsurfaces the hole extends in a direction perpendicular to the proximaland distal generally planar surfaces.
 3. The transducer of claim 2wherein the generally cylindrical outer surface and the hole aretapered.
 4. The transducer of claim 3 wherein the distal surface has adiameter of approximately 10.2 mils, the proximal surface has a diameterof approximately 9.4 mils, and the thickness of the body isapproximately 3.8 mils.
 5. The transducer of claim 4 wherein the holehas a diameter of approximately 2.5 mils at the distal end surface and adiameter of approximately 3.3 mils at the proximal end surface.
 6. Thetransducer of claim 1 wherein the low resonance frequency isapproximately one-half of the high resonance frequency.
 7. Thetransducer as in claim 6 wherein said low frequency resonance is at 6.5MHz and the high frequency resonance is at 15.0 MHz.
 8. The transducerof claim 7 wherein the transducer produces a beam having a width ofapproximately 60° at the low frequency resonance and a width ofapproximately 30° at the high frequency resonance.
 9. The transducer ofclaim 1 wherein piezoelectric material has a high dielectric content,low aging characteristics, excellent coupling characteristics and a highstrain constant.
 10. The transducer of claim 9 wherein the piezoelectricmaterial is EC-98.
 11. A transducer assembly comprising a cylindricaltip of 0.018" or less in diameter and having an outwardly facingcup-shaped recess provided therein, an ultrasonic transducer formed of apiezoelectric material disposed in the recess, said body being in theform of an annulus having a centrally disposed hole extendingtherethrough and an outer diameter and proximal and distal parallel,generally planar surfaces and having a thickness of T extending from theproximal to the distal surfaces and a wall width of W extending from theouter margin of the hole to the outer diameter, adhesive means securingsaid body in said recess, a first conductor extending through the holeand being bonded to the distal surface, and a second conductor bonded tothe proximal surface, the transducer being capable of operating at a lowresonance frequency determined by the diameter D and an aspect ratio D/Tand at a high resonance frequency determined by the thickness T and anaspect ratio W/T.
 12. The assembly of claim 11 wherein the aspect ratioW/T is equal to approximately 0.6.